Method of determining a motion vector for deriving motion vectors of a bi-predictive image block

A method of determining a motion vector for deriving motion vectors of a bi-predictive block in a current picture based on a motion vector of a co-located block in a first reference picture performed by a moving picture coding system, the method comprising:

• selecting, by the moving picture coding system, a list 0 motion vector of the co-located block in the first reference picture as the motion vector for deriving list 0 and list 1 motion vectors of the bi-predictive block if the co-located block has both a list 1 motion vector and the list 0 motion vector, the first reference picture being a type of reference picture permitted to be located temporally before the current picture when the first reference precedes the current picture and permitted to be located temporally after the current picture when the first reference picture follows the current picture.